The field of the invention is magnetic resonance angiography (xe2x80x9cMRAxe2x80x9d), and particularly, studies of the human vasculature using contrast agents which enhance the NMR signals.
Diagnostic studies of the human vasculature have many medical applications. X-ray imaging methods such as digital subtraction angiography (xe2x80x9cDSAxe2x80x9d) have found wide use in the visualization of the cardiovascular system, including the heart and associated blood vessels. Images showing the circulation of blood in the arteries and veins of the kidneys and the carotid arteries and veins of the neck and head have immense diagnostic utility. Unfortunately, however, these x-ray methods subject the patient to potentially harmful ionizing radiation and often require the use of an invasive catheter to inject a contrast agent into the vasculature to be imaged.
One of the advantages of these x-ray techniques is that image data can be acquired at a high rate (i.e. high temporal resolution) so that a sequence of images may be acquired during injection of the contrast agent. Such xe2x80x9cdynamic studiesxe2x80x9d enable one to select the image in which the bolus of contrast agent is flowing through the vasculature of interest. Earlier images in the sequence may not have sufficient contrast in the suspect vasculature, and later images may become difficult to interpret as the contrast agent reaches veins and diffuses into surrounding tissues. Subtractive methods such as that disclosed in U.S. Pat. No. 4,204,225 entitled xe2x80x9cReal-Time Digital X-ray Subtraction Imagingxe2x80x9d may be used to significantly enhance the diagnostic usefulness of such images.
Magnetic resonance angiography (MRA) uses the nuclear magnetic resonance (NMR) phenomenon to produce images of the human vasculature. When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or xe2x80x9ctippedxe2x80x9d, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins, and after the excitation signal B1 is terminated, this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx Gy and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. Each measurement is referred to in the art as a xe2x80x9cviewxe2x80x9d and the number of views determines the resolution of the image. The resulting set of received NMR signals, or views, or k-space samples, are digitized and processed to reconstruct the image using one of many well known reconstruction techniques. The total scan time is determined in part by the number of measurement cycles, or views, that are acquired for an image, and therefore, scan time can be reduced at the expense of image resolution by reducing the number of acquired views.
The most prevalent method for acquiring an NMR data set from which an image can be reconstructed is referred to as the xe2x80x9cFourier transformxe2x80x9d imaging technique or xe2x80x9cspin-warpxe2x80x9d technique. This technique is discussed in an article entitled xe2x80x9cSpin-Warp NMR Imaging and Applications to Human Whole-Body Imagingxe2x80x9d, by W. A. Edelstein et al., Physics in Medicine and Biology, Vol. 25, p. 751-756 (1980). It employs a variable amplitude phase encoding magnetic field gradient pulse prior to the acquisition of NMR signals to phase encode spatial information in the direction of this gradient. In a two-dimensional implementation (2DFT), for example, spatial information is encoded in one direction by applying a phase encoding gradient (Gy) along that direction, and then a signal is acquired in the presence of a readout magnetic field gradient (Gx) in a direction orthogonal to the phase encoding direction. The readout gradient present during the spin-echo acquisition encodes spatial information in the orthogonal direction. In a typical 2DFT pulse sequence, the magnitude of the phase encoding gradient pulse Gy is incremented (xcex94Gy) in the sequence of views that are acquired during the scan. In a three-dimensional implementation (3DFT) a third gradient (Gz) is applied before each signal readout to phase encode along the third axis. The magnitude of this second phase encoding gradient pulse Gz is also stepped through values during the scan. These 2DFT and 3DFT methods sample k-space in a rectilinear pattern.
To enhance the diagnostic capability of MRA a contrast agent such as gadolinium can be injected into the patient prior to the MRA scan. As described in U.S. Pat. No. 5,417,213 the trick with this contrast enhanced (CE) MRA method is to acquire the central k-space views at the moment the bolus of contrast agent is flowing through the vasculature of interest. Collection of the central lines of k-space during peak arterial enhancement is key to the success of a CEMRA exam. If the central lines of k-space are acquired prior to the arrival of contrast, severe image artifacts can limit the diagnostic information in the image. Alternatively, arterial images acquired after the passage of the peak arterial contrast are sometimes obscured by the enhancement of veins. In many anatomic regions, such as the carotid or renal arteries, the separation between arterial and venous enhancement can be as short as 6 seconds.
The short separation time between arterial and venous enhancement dictates the use of acquisition sequences of either low spatial resolution or very short repetition times (TR). Short TR acquisition sequences severely limit the signal-to-noise ratio (SNR) of the acquired images relative to those exams in which longer TRs are possible. The rapid acquisitions required by first pass CEMRA methods thus impose an upper limit on either spatial or temporal resolution.
Successful CEMRA of the abdomen requires that the scan be completed in a single breath-hold to limit respiratory artifacts. In conventional xe2x80x9cFourierxe2x80x9d imaging, images of high resolution and large field-of-view (FOV) can be acquired quickly in the readout gradient direction, but spatial resolution and FOV in the other two dimensions are proportional to the number of phase encoded views acquired. Clinical MRA of the abdomen relies on a scout scan to properly identify a very limited region of interest, which is then acquired with non-isotropic resolution. This method increases the time and skill necessary to perform the exam. In addition, the non-isotropic resolution and the limited FOV of the acquired images can also restrict the possibilities for post-processing the data set.
As indicated above, the acquisition of MRA data is timed such that the central region of k-space is acquired as the bolus of contrast agent arrives in the arteries of interest. The ability to time the arrival of contrast varies considerably and it is helpful in many applications to acquire a series of MRA images in a dynamic study which depicts the separate enhancement of arteries and veins. A temporal series of images is also useful for observing delayed vessel filling patterns caused by disease. This requirement has been partially addressed by acquiring a series of time resolved images using a 3D xe2x80x9cFourierxe2x80x9d acquisition as described by Korosec F., Frayne R, Grist T., Mistretta C., xe2x80x9cTime-Resolved Contrast-Enhanced 3D MR Angiographyxe2x80x9d, Magn. Reson. Med. 1996; 36:345-351 and in U.S. Pat. No. 5,713,358. However, with this method, the increased sampling rate of the center of k-space reduces the spatial resolution of the individual images in the time resolved series to about 75% of the resolution obtained when a single timed image is acquired during the passage of contrast.
There has been extensive recent work using multiple receiver coil arrays to increase imaging speed. In the SMASH technique described by Griswold, et al., xe2x80x9cSimultaneous Acquisition Of Spatial Harmonics (SMASH)xe2x80x9d Magnetic Resonance In Medicine 1999, Jun; 41(6):1235-45, multiple coils are carefully positioned in one of the Fourier phase encoding directions. Using knowledge of the coil sensitivities non-acquired phase encodings can be synthesized, thus increasing the rate at which images of a given resolution can be acquired, or increasing the resolution of images acquired at the same rate.
Another technique that can utilize arbitrary configurations of receiver coils is the SENSE technique described by Pruessmann et al., xe2x80x9cCoil Sensitivity Encoding For Fast MRIxe2x80x9d, MRM 42:952-962 (1999). This technique can be viewed as a method for exploiting the benefits of small FOV imaging while imaging a large FOV. Basically, large phase encoding steps corresponding to a small FOV are used. This causes aliasing of signals from outside the supported FOV into the small supported FOV. The signal received by each of the receiver coils corresponding to overlapping aliased voxels in the image is a linear sum of the signals emanating from these voxels multiplied by the coil sensitivity for each of the voxels. By solving the linear equations provided by n receiver coils, n overlapping voxels can be separated, providing a maximum gain in speed by a factor of n. In practice the gain in speed is usually limited to a factor of two to three because of limitations set by the coil sensitivity profiles and noise considerations.
The SMASH and SENSE methods are characterized by a factor R representing the speed increase on the order of 2 to 3 for a given resolution and a factor g, on the order of 1-1.2 representing the increase in noise beyond what would be expected for a given imaging time.
There has also been recent work using projection reconstruction methods for acquiring MRA data. Projection reconstruction methods have been known since the inception of magnetic resonance imaging. Rather than sampling k-space in a rectilinear scan pattern as is done in Fourier imaging and shown in FIG. 2, projection reconstruction methods sample k-space with a series of views that sample radial lines extending outward from the center of k-space as shown in FIG. 3. The number of views needed to sample k-space determines the length of the scan and if an insufficient number of views are acquired, streak artifacts are produced in the reconstructed image.
Efforts have been made to acquire CEMRA images in shorter scan times using undersampled projection reconstruction scanning methods. A method for reducing the number of projections in a 3D acquisition by a factor of two has been reported by F. Boada, J. Christensen, J. Gillen, and K. Thulborn, xe2x80x9cThree-Dimensional Projection Imaging With Half The Number Of Projectionsxe2x80x9d, MRM 37:470-477 (1997). In this method the acquisition is considered to occur over the upper and lower halves of a sphere using partial echoes. The projections associated with the lower half of the sphere are situated at angles intermediate between those of the upper half. A half Fourier algorithm is used to synthesize the data from the missing part of each echo, thus filling in the intermediate data in each hemisphere. This technique is not really undersampling, but instead, provides the missing data through a valid synthesis of the missing echo data. However, it does provide a factor of two increase in scanning speed relative to a 3D projection reconstruction sequence employing full echoes at all projection angles.
The present invention is an improved CEMRA method which employs a three-dimensional projection reconstruction method for sampling k-space. It has been discovered that the periphery of k-space can be substantially undersampled when using a 3D projection reconstruction technique for acquiring CEMRA data. Artifacts which are to be expected in the reconstructed images are surprisingly unobtrusive and undersampling by factors far in excess of 2 results in the acquisition of clinically useful images in less scan time.
Another aspect of the invention is the use of a 3D projection reconstruction acquisition method for the acquisition of a series of images in a dynamic CEMRA study. The acquisition of each image frame in the series can be shortened by undersampling the periphery of k-space. This undersampling increases the time resolution of the dynamic study without a loss in image resolution.
Yet another aspect of the present invention is a method for removing artifacts from images acquired in a dynamic CEMRA study using an undersampled, 3D, projection reconstruction acquisition. Successive image frames in the dynamic study are acquired with a different set of projection angles, or views. These are combined to form an image data set that completely samples k-space. Each set of projection views undersamples the peripheral region of k-space, but a complete image data set can be formed by combining peripheral k-space data acquired during adjacent image frames with the central region and peripheral region k-space samples at the desired time during the study.
The foregoing and other objects and advantages of the invention will appear from the following description. In the description, reference is made to the accompanying drawings which form a part hereof, and in which there is shown by way of illustration a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention, however, and reference is made therefore to the claims herein for interpreting the scope of the invention.